Low-temperature fluid container



March 14, 1961 A. PASTUHOV ET AL 2,974,496

LOW-TEMPERATURE FLUID CONTAINER ,2 Sheets-Sheet 1 Filed Feb. 10, 1958 IN V EN TORS A lexis Das fuhov George V ,Qobl'nson, Jr.

ATTORNEYJ nited State 2,974,496 LOW-TEMPERATURE FLUID CONTAINER Filed Feb. 10, 1958, Ser. No. 714,130

Claims. (Cl. 62--45) This invention relates to the storage and transportation of liquefied gases at low temperatures, and it relates more particularly to insulated containers of large capacity which either are not filled to maximum capacity or else in which the level of the liquid in the container has been reduced because of the loss of liquid by vaporization or by the removal of liquid for use.

Many types of containers suitable for handling liquids to be maintained at relatively low temperatures are known. One of the most common of these is the metal tank housed completely within an outer tank or receptacle with thermal insulating material packed to fill the space between the inner metal and the outside receptacle, or else in which the space between the tank shells is evacuated to cut down heat loss. Alternatively, the metal tank may be placed in a receptacle formed of such material as cellular glass, foamed glass, balsa wood, or cork, which serves principally as an insulation to cut down heat loss and which may incidentally also serve as a container for the liquid in the event of failure of the metal tank. Because the tank in direct contact with the liquid is formed of a metal, it will be subject to high thermal expansions and contractions when exposed to temperature gradients or temperature changes which may range from ambient temperature, on the one hand, and a temperature as low as 250 F. on the other hand, as when the container is used for the storage and transportation of a liquefied natural gas at about atmospheric presure.

When a tank designed to contain liquefied gases is not a completely filled, there exists above the liquid level in the tank a gas blanket or vapor space which at first may consist chiefly of the residual gas or air left in the tank but terized by a low' heat conductivity coefficient, so that a relatively large temperature differential will usually exist throughout the length of the gas column with a minimum temperature at the liquid-gas interface to a maximum temperature at about the upper end of the gas column. Because metals suitable for tank-wall construction have relatively low thermal conductivity at the low temperatures involved, the tank walls themselves will also have a' temperature gradient in the areas surrounding the gas column, the temperatures of which will approximate that t .of: the gas column that is surrounded. Such temperature differential through the length of the metal forming the walls of the tank will result in the development of induced stresses which lead to deformation or failure of the tank.

- For example, in a tank containing liquid oxygen (B.P. 297.4 F.) the temperature of the oxygen gas at the top of a tank fifty feet high may be in the order of zero detent O M.

7 2,974,496 Patented Mar. 14, 196:1

grees F. when the tank has about one foot of liquid oxygen, while the gas at the liquid-gas interface will be about 297 F. Such a wide temperature gradient of 297 distributed through the forty-nine feet of the tank can cause the build-up of a stressed relationship in the metal walls of the tank that can lead to fracture or deformation. Thus there is a need for a means to eliminate these induced stresses or else to minimize the forces causing the build-up of stresses so as to prevent deterioration or destruction of the tank. This can be done by the elimination or material reduction of the temperature gradients which give rise to the thermal stresses. Thus, it is primarily desirable to minimize the temperature gradient existing through the gas column and hence to minimize the thermal stresses induced in the portions of the metal tank surrounding the gas column.

It is therefore an object of this invention to provide means for reducing the temperature gradients within a tank housing a cold-boiling liquefied gas, and it is a related object to minimize the temperature gradient through the area of a tank above the portion filled with a low-boiling liquefied gas.

It is another object of this invention to reduce or to eliminate the development of thermal stresses in the walls of a metal tank partially filled with a liquefied gas at low temperature.

These and other objects and advantages of this invention will hereinafter appear, and, for purposes of illustration, but not of limitation, an embodiment of the invention is shown in the accompanying drawing, in which Figure 1 is a vertical cross-sectional view of a tank construction embodying the features of this invention;

Figure 2 is a horizontal cross-sectional view of the metal tank shown in Figure 1 taken along the line 22 of Figure 1;

Figure 3 is a typical curve showing the rise in temperature through the gas column as related to the distance from the liquid-gas interface;

Figure 4 is a curve similar to that of Figure ,3 showing the temperatures in stainless steel and aluminum as reflected from the temperature gradient of the gas collumn in contact therewith; and

Figure 5 is a vertical sectional view of a tank construction similar to that of Figure 1 showing a modification in the tubular heat transfer members.

By the process of this invention, the temperature gradient, and therefore the thermal stresses, within an involved portion of a metal stroage tank for liquefied gases, are minimized by'the concept of suspending one or more metal conductors of high thermal conductivity lengthwise through the gas column, preferably from the top of the tank through the gas column and into contact with the liquefied gas stored in the tank. It is believed that the described metal conductors of high thermal conductivity are capable of functioning as condensers to extract heat rapidly from the gas in the gas column for flow downwardly through the conductor toward the liquid column wherebythe temperature difierentials through the gas column are substantially minimized. Thus, in providing an elficient heat path, the temperature in the upper portion of the gas column, as well as throughout the length of the gas column, can be maintained at levels not too much greater than the temperature of the liquid content material. The metal conductors of high heat conductivity may be embodied in the construction in the form of metallic bars, strips, or tubing, or in the form of a perforated tubing, serrated sheets, and the like, which may 3 or may not be equipped with fins or fin-like'attachments for materially increasing the area of contact between the gases or vapors and heat conductors without materially increasing the volume of space occupied by the metal conductors, thereby to enable the high capacity of the tank to be maintained.

The concepts of this invention may be more fully illustrated with reference to the figures of the drawing. There is represented in Figure 1 a vertical cross-sectional view of a metal tank having side walls '10, a top Wall 11, and a bottom wall 11a. The entire tank structure is surrounded by a relatively thick layer 12 of insulation which serves primarily to minimize heat loss into the tank and which may serve incidentally as an outside container for confining the liquid in the event of failure of the metal tank, especially when the heat-insulating material is formed of such materials as balsa wood which provide pockets for the build-up of pressures that militate against penetration of the liquid content material through the insulation layer. Instead, an outer tank may be employed about the insulating layer to provide the additional confinement when desired. A column of liquefied gas 13 is housed in the lower end portion of the tank. The space 14 above the liquid will be filled with the vapors released from the liquid by vaporization of the liquefied gas as brought about by the inevitable heat leaks from the outside atmosphere. The liquid-gas interface is represented by line 15, and up to the interface the walls of the metal tank will generally correspond to the temperature of the liquefied gas in surface contact therewith. Above the liquid the temperature of the metal walls of the tank will be at higher temperatures, depending upon the heat conductivity of the metal of which the tank is formed and the temperature of the vapors at the corresponding levels in the vapor-filled portion of the tank.

To minimize the temperature differential in the vaporfilled portion or column 14, use is made of metal condoctors 16 of high thermal conductivity which reach from the tank top 11 and extend downwardly substantially throughout the entire length of the gas column 14, and preferably to the liquid-gas interface 15 and into the liquid column 13. The tank, as illustrated in Figure 1, may further be equipped with a filling line 20 which makes use of a pressure-regulating valve 17 to control the pressure of the vapors within the tank and through which excess vapors can be vented. The tank can be further provided with a draw-off 'line 19 incorporating a valve 18. It will be understood that it is not essential for the heat conductors 16 to extend downwardly into the liquid in the tank, but it is preferred to have the metal conductors extend into the liquid so as to make the minimum temperatures of the liquid available "to .the conductors for temperature distribution.

In Figure 2, wherein like numerals refer to like components in Figure 1, there may be seen one possible arrangement of a series of a plurality of bars 16 employed as thermal conductors and distributed in a desirable arrangement through the gas column, in accordance with the concepts of this invention. It will be understood that a greater number of bars may be employed, and that the bars may be distributed in different arrangements to achieve the desired degree of temperature control.

However, even though some differences will exist in the thermal conductivities of metal, it will be seen that the relatively cross-sectional areas of the metal walls forming the tank cannot by themselves compensate for the temperature gradients in the gas column, since the relatively low thermal conductivity of the metal walls of the tank will militate against their functioning as the sole condensers. On the other hand, the tank-wall temperatures closely approximate the temperature of the enclosed gas column. Thus there arises the necessity for materially reducing the temperature gradient in the column, thereby materially to decrease the thermal stresses induced into the tank walls.

The thermal conductors are preferably formed of such high heat-conductivity materials as represented by copper or aluminum which exhibit the best thermal conductance at the low-temperature conditions existing. The length of each conductor is preferably formed so that its free end will engage or preferably extend into the liquid column. The total cross-sectional area of the conductors should be calculated to afford suflicient heat-transfer area efliciently to remove heat of the gas from the gas column, and thereby to minimize the temperature gradient between the top and the bottom of the gas column. The desired cross section may be supplied by a single conductor but it will be preferred to make use of a plurality of conductors fairly uniformly distributed through the interior of the tank for optimum removal of heat and for the maintenance of a minimum temperature gradient from the top to the bottom of the tank. The thermal conductors may be fixed to the top of the tank by any suitable means. The connection may be rigid, such as effected by welding, or the connection may be a pivotal connection to enable the conductors to swing with some freedom, such as by a ball-joint connection.

The process of this invention thus provides a means for reducing the temperature gradient in a column of gas generated from a low-boiling liquefied gas housed within a metal tank. In reducing the temperature gradient in the gas column and hence in the walls of the tank surrounding the gas column, the thermal stresses in the tank walls will be minimized so as to make it possible to carry or to house any quantity of the liquefied gas in the con- Figure 3 illustrates the general type of temperature gradient which has been calculated to exist ina gas column above the liquid-gas interface in a tank containing a liquefied natural gas. In Figure 4, illustration is made of the manner in which stainless steel and aluminum, when employed as walls of the tank, tend to approximate the curve of the temperature gradient in'the gas column, as illustrated in Figure 3. It will be observed that for a given gas-column height, corresponding to the height of the tank wall of Figure 4, the temperature gradient for aluminum will be less than that for stainless steel. 'Thisdifferential will be due to the differences in thermal conductivity of these metals at the temperatures involved.

tainer, even though the quantity may vary and even though the quantity does not represent the full capacity of the tank.

It will be understood that changes may be made in the details of construction, arrangement, and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. In a tank structure for housing a low-boiling liquefied gas which sometimes partially fills the tank comprising a housing of metal walls which are subject to expansions and contractions in response to temperature change in the walls thereof, a thermal insulating material of low heat conductivity about the outer walls of the tank to minimize heat leaks, a low-boiling liquefied gas housed within the tank subdividing the housing into an upper vapor column and a lower liquid column, and a plurality of elongate elements of good heat conductivity extending vertically in closely spaced apart relation through the vapor column formed above the liquefied gas in the tank with the lower end portion extending into the liquid column to achieve substantial equalization of temperature through the vapor column and between the liquid column and the vapor column.

2. A tank structure as claimed in claim 1 in which the thermal conducting means comprises elongate elements formed of copper extending lengthwise through the vapor column in the housing.

3. A tank structure as claimed in claim 1 in which the thermal conducting means comprises elongate elements formed of aluminum extending lengthwise through the vapor column in the housing.

formed of a metal of heat conductivity extending lengthwise through the vapor column.

5. A tank structure as claimed in claim 1 in which thermal conducting means comprises perforated tubular members formed of a metal of good heat conductivity extending lengthwise through the vapor column in the housmg.

References Cited in the file of this patent UNITED STATES PATENTS Jefferson Jan. 30, 1912 Hooper et al Mar. 26, 1940 Schilling July 4, 1950 FOREIGN PATENTS Germany Aug. 19, 1932 

